Exam II Review MSE

Diffusion

• mass transport by atomic motion

• for gases/liquids = random (Brownian) motion

• for solids = interstitial/vacancy diffusion

• important for electrical conductivity, case hardening, doping

Fick’s First Law

• diffusion flux is constant = steady state

• ex: helium blimp

Fick’s Second Law

• diffusion flux is NOT constant = non-steady state

• ex: case hardening, doping

Trends of Diffusion

• higher temperature = faster diffusion

• smaller species diffuse faster

• more open structures = faster diffusion

• materials with secondary bonding = faster diffusion

Why does diffusion occur?

• driven by thermodynamics; primary reason is entropy, but also enthalpy to minimize free energy

Phase

• chemically uniform, physically distinct; often mechanically separable

• ex: salad dressing

Equilibrium

• minimum energy state for a given temperature, pressure, and composition

• an equilibrium phase will stay constant over time

Unary Phase Diagrams

• consists of single phase

• ex: water

Binary Isomorphous Phase Diagrams

• two components; complete liquid and solid solubility

• ex: Cu-Ni phase diagram

Meta-Stable State

• a system that is trapped (for a certain amount of time) in a phase that is not at minimum energy

Binary Eutectic Phase Diagrams

• two components; “easily melted”

• Eutectic = liquid phase separates into two different solid solutions

• ex: Pb-Sn/Cu-Ag phase diagram

Congruent Transformation

• no change in composition upon phase transformation

• liquid to one solid solution

Incongruent Transformation

• phase transformation where at least one of the phases go through composition change

• solid solution to another solution plus liquid

Intermetallic Compounds

• discrete metal compounds rather than solutions; distinct chemical formulas

• usually straight vertical line on phase diagram

Intermediate Phases

• solid solutions that do not extend to pure components in the phase diagram

Eutectoid Reaction

• one solid phase turning into two other solid phases upon cooling

Peritectic Reaction

• one solid phase transforms into liquid and a different solid phase upon heating